1. Field of the Invention
This invention relates to a device for storing data in digital form, and particularly to a data storage device having a digital data erasing function.
2. Description of Related Art
Digital data storage devices are known for storing both voice digital data and image digital data. For example, a facsimile machine equipped with an automatic telephone message recorder (automatically answering telephone) is disclosed in the U.S. Pat. No. 5,018,186 and Japanese Unexamined Patent Provisional Publication No. 1-286671, in which image data are received by the facsimile machine and voice data are received by the automatically answering telephone. The received data are stored in a data storage device provided in the facsimile machine.
In such a storage device, when a storage area is filled with data, no more digital data can be stored in the data storage device. Therefore, digital data which have been already stored in the storage device must be erased. In order to satisfy this requirement, the conventional data storage device is so designed that the voice digital data is erased on the basis of an erasing instruction and the image digital data is erased when it is output from the device.
If an object to be erased (an erasing target) can be erased on the basis of the erasing instruction for the voice digital data as described above, the following actions can be performed. That is, in a case where both the voice digital data and the image digital data are stored in a common storage means, it is possible that only the voice digital data is erased while keeping only the image digital data. In addition, in a case where the voice digital data and the image digital data are respectively stored in different storage means, it is possible for the voice digital data to be erased when the storage means for the voice digital data is filled with the voice digital data, thereby freeing storage for the newly arriving voice digital data.
On the other hand, the image digital data is erased only when it is output, and this limitation to erasure of the image digital data produces problems. For example, in the case where both of the voice digital data and the image digital data are stored in a common storage device, if the storage area is filled with the image digital data and the image digital data cannot be output due to a lack of recording sheets or a lack of recording toner, the voice digital data must be erased to store newly arrived image digital data. Thus, voice digital data that should be kept must be erased.
Further, in the case where the voice digital data and the image digital data are stored in the different storage means, there is no requirement to erase voice digital data in order to secure storage area for the arriving image digital data, but no storage area for storing new arrived image digital data is available unless the image digital data is output.